I've played with a lot of different GMs over the years, covering the entire range of potential GMs. I'd say approximately 15 GMs locally over the last 28 years, including friends.
The worst three;
Old Friend: Railroader with no sense of reasonability.
a> Party is harried by a Wizard named "Synch O'Phant". Yes, it was that bad. Guy used to pop in at random, blast the party and make his escape. Over and over without any apparent reason. Always some kind of Deux Ex Machina escape, even from a grapple, even when we "killed" him.
b> Kingdom of 1300 people. Capital city of 1000, small farming town of 300 located several hundred miles away. Nothing else anywhere around. Then we encounter a "patrol" of 100 soldiers. Then we find several hundred guards and soldiers in the capital city.
c> We decide to leave this dusty, useless port (the above capital). No ship will transport us for any price. We cannot buy a ship. We attempt to steal one. The entire crew is on deck, armed to the teeth, at the end of the first round. In the middle of the night Entire galleys of soldiers are making their way across the harbor and boarding us in a single round. The ship starts on fire. We're down to a handful of hitpoints, but it's obvious that the GM WILL NOT KILL US. So we keep fighting. GM says "The ship has burned to the water line and is about to sink. What do you do?" We wad up character sheets, throw them at him and scream "we drown!". End of game. For years afterward he cannot understand why people keep making excuses not to play in his next game.
Random Kid: At a FLGS many years past. Something like 18 players in the next game over, this kid struggling to get any players. Me and one other guy agree to play. His "world" consists of ONE city, ONE road and ONE dungeon. We are attacked left and right, cannot even get past the first encounter at the dungeon, so we retreat to the city. Everyone, and I mean *everyone* including the guards themselves, are harassing us and attempting to rob us. We go to the ONE Inn. Innkeeper demands a fortune for a crappy, filthy room, threatens to kill us if we don't pay. Other player wisely decides he has other things to do and walks away. I stay for more of this abuse for another hour, trying to talk to the boy about his crappy, abusive world and why anyone would want to play with him. He doesn't get it. I quit and walk away, causing him to very nearly break down and cry. The next week he's right there again trying to recruit new players and getting upset that I and a good half-dozen other people intercede anytime he hooks a new victim.
My rules, my way" Very recently, I responded to a players wanted ad for a local game. The GM seemed happy, initially anyways, for me to join. Was a bit tricky on getting me the address, which was my first clue. When I show up, he seems surprised and says "I didn't think you would show up". Three times in the first half hour. Then goes on to tell me how much of a stickler for the rules he is, telling me he's very firm on the Frostburn rules for extreme cold and he's going to watch my Wizard's material components like a hawk. He tells me a tale of a past player who kept casting a minor spell that required a component that could not be obtained in the cold lands he was playing in, so he never let that player cast that spell again. Ok, so I take Eschew Materials. His angry glare could have melted the arctic icepack. Next we're running a 4 player 1st level party (Cleric, Rogue, Monk and Wizard) through an adventure WAY TOO TOUGH for our group. When I start figuring things out, he accuses me of having read the adventure. Then Mr. I-Own-every-rulebook-in-existence-and-will-keep-you-to-it makes a silly mistake on a Tumble check. Then he kills my character and very nearly TPKs the entire group until pulling a Deux Ex Machina save out of his ass. Later on, I e-mail him a new character and point out the tumble error. Mr. IOERIEAWKYTI angrily responds that I am a "rules lawyer" and that he prefers to play under more loose rules. Which makes no sense given how he so adamantly lectured me at the beginning of play about how much of a stickler for the rules he is. GOOD RIDDANCE.